One of the major issues of optical microresonator devices is that their operating wavelength (or conversely, frequency) is temperature sensitive, making it necessary to temperature control the device for many practical applications. As an example, consider an optical microresonator 140 coupled to a single optical waveguide 130 (shown in FIG. 1), with an input optical signal 110 and an output optical signal 120; assume that the coupled optical microresonator 100 has a bandwidth of 0.05 nm, a Free-Spectral-Range (FSR) of 1 nm, and an operating wavelength close to 1550 nm. An example optical loss spectrum for such a microresonator is shown in FIG. 2 for a specific ambient temperature, e.g. room temperature—the resonances occur over a wide wavelength range, e.g. 100 nm or more. When the temperature is increased or decreased, the effective index of the optical waveguide making up the microresonator changes, therefore changing the effective length of the microresonator, which then changes the wavelengths of the microresonator resonances. This is shown in FIG. 3, which shows the resonances ‘tuned up’ by 0.3 nm due to heating. It is in fact this thermal variation of the microresonator, or tuning, of its resonances, that is used in the tunable time delay described in the original patent (U.S. Pat. No. 7,831,119 B2) that this new concept builds upon. By changing the temperature of the microresonator, the resonance wavelength can be tuned considerably, such as shown in FIG. 4—it can be tuned beyond a single FSR; this allows a resonance to be moved to any optical wavelength within a broad operating wavelength range by first choosing the closest resonance, and then tuning that resonance to the required operating wavelength by changing its temperature.
An example of an optical time delay device incorporating a series of independent optical microresonators is shown in FIG. 5. The optical time delay chip includes multiple optical microresonators, similar to those shown in FIG. 1, coupled to a single optical waveguide; the number of microresonators is typically an even number, varying from a small number such as 10 up to a large number greater than 100. The output signal of the device includes the effects of the optical resonances of each of the microresonators acting on the device input signal. The even number of microresonators is split into two sets of microresonators for use in the Balanced SCISSOR approach (U.S. Pat. No. 7,831,119 B2), which provides tunable optical delay with wide bandwidth and low distortion to a signal. The two sets of microresonators are initially aligned with the signal wavelength, which is achieved by first setting the overall chip temperature, so that all of the microresonators provide resonances that are below the operating wavelength of the signal (FIG. 6, 610), then the heaters on each of the microresonators are used to tune their resonances up to the operating wavelength of the signal, in this case at 1550 nm, 620. The Balanced SCISSOR concept then tunes one set, comprised of half of the microresonators, up in wavelength and one set, comprised of the other half of the microresonators, down in wavelength at the same time to produce a variable time delay, which is dependent on the wavelength offset of the two sets of microresonators, 630.
The Balanced SCISSOR approach requires that the overall chip temperature be held at a fixed temperature, typically by using a Thermo-Electric-Cooler (TEC) to heat or cool the device, plus a thermistor on or close to the device to measure its temperature and provide feedback to control the TEC. If the tuning range of the microresonators is less than the FSR, then in order for the device to be able to operate at any wavelength it is necessary to change the temperature of the entire chip to first place the resonances in the correct position relative to the operating wavelength, before the microresonators are then tuned to the operating wavelength. The use of a TEC to control the temperature of the device adds significant power dissipation to the overall packaged device, as well as additional size and cost. If at all possible it would be of great utility to remove the need to use a TEC for correct device operation.